1. Field of the Invention
The present invention relates to an image forming process and an image forming apparatus, having a toner carrier which faces an electrostatic latent image carrier and carries toner to the developing zone thereon, and provided with a refresh means which refreshes the toner on the toner carrier when image transfer to a recording medium is not done or when the apparatus is in a state of not transferring image.
2. Description of the Related Art
Generally, by repetition of image forming with a developing apparatus which uses uni-component developer, toner of small grain size sticks to the surf ace of the toner carrier attracted by mirror-image force caused by high electrification thereof, the stuck toner hinders another toner from being charged by frictional contact with the surface of the toner carrier, and toner may be carried as far as the latent image on the electrostatic latent image carrier without being charged uniformly on the toner carrier.
This may cause problems such as light density of image and "fog".
The phenomenon like this is furthered in a case where the image to be formed is of low print rate, because toner particles are liable to remain on the toner carrier (developing sleeve), for small quantity of toner jumps from the toner carrier to the electrostatic latent image zone on the image carrier (photoreceptor drum), effecting reduction in print density and "fog".
It is possible, in these cases, to alleviate the phenomenon by letting the toner jump in quantity from the developing sleeve to the photoreceptor drum by performing image forming of high print rate image pattern such as black solid fills and transferring the toner to the recording medium to consume the toner on the developing sleeve; however, in a case where black solid fills print is not done after the apparatus is left standing without consuming the toner for an extended period, the toner particles, affected by humidity and others, adheres to the surface of the developing sleeve and may not be able to be recovered. The phenomenon is liable to occur especially under high humidity conditions.
Heretofore, studies on the shape of surface and material of the toner carrier have been carried on, improvements have been made to make enough use of the capabilities of charging toner; however, in some cases toner is charged excessively and the phenomenon described above is easy to occur.
In the developing zone, toner reciprocates between the electrostatic latent image and the toner carrier by applying an AC bias.
Accordingly, by applying a higher AC bias when developing is not executed than when development is executed, the toner is removed from the toner carrier and allowed to jump to the electrostatic latent image.
Therefore, in cases where the print rate is low, that is, the number of dots of the image is small, deterioration in transferred image is prevented by applying an alternating electric field to the toner carrier to consume the toner thereon after the formation of the image, by which the surface of the toner carrier is refreshed.
However, there are drawbacks that the effect is not expected unless applying period of the alternating electric field is long, because, after the apparatus is left standing over an extended period of time, the toner is removed in consecutive order from the surface layer, and a lot of the toner is consumed, which means it is not efficient and economical.
Further, in some cases, as the alternating electric field is applied to the toner carrier and the electrostatic latent image zone when refresh is performed, the reverse-charged toner in the a developing device attaches to the transfer part and pollute the reverse side of the printing sheet. In high humidity condition the reverse-charged toner may be mixed in the positive-charged toner, which raises the like problem as mentioned above.